Robert Morris

Oakland, California - December 15th 2020


I gratefully acknowledge Dave Jackson for actually getting me to write this and Cornell Wright, III for the discussions with me and comments.  All errors are mine.

Sequestering Agricultural Carbon in Deep Anoxic Zones

Low risk CO2 sequestration technology with a pathway to $10 / ton cost and geologic time scale.

Putting Coal Back in the Ground  

At my former company, TerrAvion, I was thinking about soil sequestration of carbon, of which peat bogs are the logical end point--realizing that was not a big enough opportunity to meet our climate goals, I then started thinking about Black Sea shipwrecks, inspired by the carboniferous period.  It made me realize that the best alternative for carbon sequestration that meets our requirements [below] is to use the only 40 Gt / year carbon fixing industry we have now--agriculture--with coal’s transportation infrastructure running in reverse, to take the formerly atmospheric carbon to places with conditions that mimic the conditions that created coal fields originally.  

Characteristics of a Solution 

What we’re looking for and trying to do with carbon sequestration:

  1. Carbon must be sequestered for at least civilizational time scales, preferably geological
  2. Available scale must be greater than 10 Gt / year for more than 20 years
  3. Cost must be on a path to $10/ton with Gt scale, starts at $50 now
  4. Must not have crazy environmental side-effects
  5. Should inspire minimal geo-engineering antipathy

Basically, carbon sequestration has to be a good fraction of all historic emissions plus ongoing industrial processes it is infeasible to decarbonize.  It has to be affordable to corporations and governments, and ideally less expensive than some forms of carbon fuel.  $10 / ton might be more aspirational than actual, but it seems possible to me with sufficient scale and improvements in the infrastructure to do this.

We could put a comparison chart here, but everything I’ve read about carbon sequestration says that something along the lines of the proposal here is the only method that ticks all the boxes above, or at least the best and most accessible method of the boxes above.

How it Works

The basic idea is to buy whatever plant carbon source is super cheap and sink it in anoxic basins in the ocean.  My stand in for a decent carbon source, not having done exhaustive sourcing, is sugarcane bagasse, but really it could be corn stover, hayage, or forestry byproducts, or all of the above.  This should be sourced as close to water transportation as possible to reduce transportation costs, ideally it is also dense, with low water content, low sulfur and nitrogen, and high carbon content--but most importantly it should be cheap and unwanted for fuel or fertilizer.  

Once procured and moved to a seaport, the organic matter could be bundled like the large commercial hay bales, perhaps with brick, sand, construction by-products, or other inert, non-buoyant material to make it sink to the bottom.  At large-enough scale, one could envision a continuous process that looks like dredging in reverse that basically deposits a slurry of plant, water, and sand into the anoxic basin.  Probably, this handing and sinking problem is the key operational challenge of turning this into a scalable process.  When organic material is sunk into anoxic zone, bacteria has a really hard time breaking it down, so it just sits there for thousands of years.  All of our modern coal formed when bacteria could not break down ancient plants.  We are just going to ship loads of agricultural waste to run the process in reverse.

It seems likely that we could engineer anoxic basins as well as use naturally occurring basins.  Anoxia can be induced by large quantities of organic matter (see bogs and eutrophication) as long as circulation is low, especially if the water is warm, lowering oxygen solubility.  Given our ability to build dams and underwater structures and move large quantities of sand, it seems like we could create synthetic basins, fill them, remove or allow nature to remove the oxygen, and cover them in locations where they are unlikely to be disturbed on civilizational time scales.  Perhaps building an underwater landfill as it were, is cheaper than shipping long distances, but this is a pure cost equation unless regulatory or environmental pressure drives decisions.

Unknowns to Resolve

Demand and willingness to pay

It seems like large tech companies and certain utilities under stringent regulation are paying to sequester carbon.  What is demand and willingness to pay?

Best way to implement

This seems like something that will be solved incrementally, probably no show stoppers, but non-trivial operations questions include:

Supply Constraints

There are likely few actual supply constraints, supply is probably highly dependent on transportation costs and alternative value.  Expanding the economically feasible catchment area is probably very important to scaling supply.  My stand-in of bagasse as a feedstock could be collected at a (solar powered) sugar mill, but stover has to be purchased and has alternative uses in the field.  Over time it seems like plant breeding will help increase stover (current breeding places negative value on this) if this is truly to become a scalable solution.

Regulatory Questions

What permits are required to do this?

Proof of Sequestration

What verification is required to be credible?  What monitoring/measurement is required to be part of the “non-voluntary” market?

Environmental Concerns

Ideally we’d at least be able to claim that what we’re doing improves ocean habitat in meaningful ways and does not contribute to ocean acidification.  There will probably be some impact, but climate change does too, good to be able to quantify this and say it is minimal.

Business Model and Defensibility

Somewhat in line with the willingness to pay issue, is how does this become a business and why aren’t we suckers for figuring this out for everyone else, if this is really as great as I think it is.  I have a few basic ideas at this juncture, but there are probably others.

1st Approximations of Sea and Land Use